To answer your question more specifically, what type of photography do you do? How do you feel that your 400D holds you back as a photographer? As others have mentioned, you will likely see a greater improvement by investing in lenses, unless you have a specific reason to upgrade to the 7D.
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gjbMay 31 '11 at 8:48

7 Answers
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The 450D is of course an upgrade to the 400D and the changes/improvements between the 400D and the 7D will probably be greater than between the 450D and the 7D, so add that up while reading this.

To begin, as you most likely are aware, lenses matter more for image quality than the actual camera does. The 18MP sensor, allows me to see the details on the photos more, and how unsharp some of my lenses are (I see differences very clearly compared to what I saw with the 12MP sensor of the 450D). So, we have more pixels, not that important, but you do get more details and most importantly, you get a greater possibility to crop images. Otherwise, regarding quality of the image, there is little difference between my 7D and my old 450D. I believe the 7D has somewhat better colors though, but otherwise, I think they are almost equal.

The 7D's build quality blows the 450D away. This beast is not the kind of plastic (but expensive) "toy" that my 450D was. It feels a lot better in the hand, and I am way more confident about it being able to take some rough handling at times.

Changing settings, such as focus mode and so on, is much faster with the 7D. It is more of a "pro" camera. It also has way more options, allowing you to fine tune the image.

The autofocus is amazing. It is faster than the 450D, and it is easier to set up, and more precise. Depending on what you do, this might make the difference between a great image and a "meh" image.

It shoots series of pictures faster too, which will matter if you photograph wildlife or sports.

I should also add that the 7D handles high ISO numbers much better. This is not that important for me as I mostly use a tripod anyway, but still nice to have when you shoot without it.

It also has nice features such as a built in gyro, so you can check whether the horizon (or something) actually is straight. You also get filming, which is nice, but I have not used that for anything serious; it takes great movies though. I know you don't care, but it is nice :).

What I really like is the built in wireless flash control. I have an external 440EXII flash, and if I wanted to control it wirelessly with the 450D, I would either have had to buy a transmitter (circa 200$) or a 580EXII (which has a built in transmitter, but ain't exactly cheap either).

I went from a 350 to the 7D and another thing I found was that the clarity of the viewfinder was also a vast improvement and it still worked with my RC-1 wireless remote (unlike the 20-50D models). Great list though, covered pretty much everything I noticed as well.
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RobinJan 10 '13 at 15:11

The higher-end cameras first all have dual-control dials, this will be true if you go to a 7D or even a 60D. This and added buttons lets you operate the camera much faster since there are more direct controls and less reasons to enter the menu system.

For the 7D but not the 60D, you get a 100% coverage viewfinder. This feature alone is worth the upgrade if you care about perfect composition. Once you there, it is very difficult to go back.

There are plenty of other details which may or may not be important to you. Many appreciate the higher-end build quality, faster-speed of operation, faster drive and more.

If you shoot action or street photography, for example, the faster speed will greatly increase your chances of snaring a keeper.

The dual-axis digital level is a really nice bonus. I have a 3-axis add-on for the hot-shoe (which predates digital camera's having levels) but I find having the levels in the viewfinder much more ergonomic. I rarely use the hot-shoe one anymore.

I upgraded from 350D (Rebel XT) to the 7D. The important factors for me when upgrading were the built-in wireless flash controller, the 100% viewfinder, better controls and so much better LCD.

This is definitely a body with a pro feel. If you have bigger hands, the 7D's bigger size is very comfortable in your hand. Once I got the new camera, I completely stopped using the old one. Not even once (well, once for actually shooting my new gear, and a few times for stopping down lenses to be reversed mounted on the 7D).

My view is that if you need reasons to justify purchasing a new body, you don't really need the body (yet). To spend wisely, you'd have a problem first, then search for solution, not the other way around.

Perhaps a new lens, a flash, a sturdy tripod or an exotic trip would make more difference to your photos than a body that just seems "better" in no important way. A lens would also last more years than the body.

If you're keen on Strobist work, then the 7D allows you to trigger an external flash gun wirelessly with full E-TTL metering. You could do the same on your 400D with an ST-E2 transmitter, but that's a minor inconvenience.

The 7D (aswell as the 550D) uses a new type of sensor that uses gapless microlenses as explained briefly here DP 550D Review. So far as I can tell this results in vastly lower noise at higher ISOs (550D in comparison to my old 1000D).